Traditionally, a window covering system includes a headrail, a covering material, a bottom rail and a driving device. The covering material is positioned between the headrail and the bottom rail. The driving device is positioned in the headrail, wherein the driving device includes a spring box and a lifting cord module. The lifting cord module includes a winding spool which can be driven to rotate by the spring box or the bottom rail and a lifting cord which is wound upon the winding spool and connected to the headrail and the bottom rail, wherein the lifting cord can pass through the covering material. Alternatively, the covering material can be disposed in a space defined by the lifting cord. When the lifting cord is driven by the spring box or the bottom rail, the lifting cord can be collected by or released from the winding spool, such that the bottom rail moves toward or away from the headrail in order to control the collection or expansion of the covering material.
When the weight of the covering material and the bottom rail with the force provided by the spring box are balanced by the friction force of the whole window covering system, the bottom rail can stop at any position to retain the level of light blockage of the covering material. However, the friction force of the whole window covering system is difficult to be controlled effectively comparing to the weight of the covering material and the bottom rail. Especially, the force provided by the spring box is generated by the elasticity of a spiral spring within the spring box, wherein the spiral spring has a problem of potential elastic fatigue. In addition, the closer the bottom rail ascends to the headrail, the more covering material accumulates on the bottom rail, and hence the heavier the overall weight of the bottom rail and the covering material. Therefore, it is likely that the bottom rail would more or less descend for a distance from a desired position. In such case, it is inconvenient and annoying to anyone operating the window covering system. In respect of the above problems, a locking device should be provide to fix the bottom rail at any position, thereby the bottom rail can remain stationary regardless of gravity. However, the locking device must be unlocked to descend the bottom rail and expand the covering material. Moreover, an operable height of a window covering for different users may be different, thus a convenient unlocking method should be provided to coordinate with the locking device to ease the operation of the window covering system.